1. Field of the Invention
The present invention relates to a system for feeding solid granulated material into the reaction zone of a fluidized bed reaction system. More particularly, the invention relates to the use of a movable nozzle whereby the solid granulated material may be directed within the fluidized bed reaction zone to control mixing, heat transfer, and solid reactant residence time.
2. Description of the Prior Art
Gas-solid reaction systems employing a fluidized bed to contact the gaseous and solid reactants are an effective means to react these materials. This effectiveness is due in part to the large gas-solid surface contact area inherent in a fluidized bed reaction system. Fluidized bed reaction systems are also characterized by high rates of solids mixing in the bed reaction zone resulting in improved reaction control over fixed- or moving-bed gas-solid gas reaction systems.
Typical gas-solid reactions suitable for fluidized bed reaction systems include: calcining, in which a solid reactant is heated in order to induce a chemical or physical change; drying, in which a moisture bearing solid is heated in order to drive off the moisture; and combustion, in which a solid fuel such as coal is reacted with an oxygen bearing gas for the purpose of producing useful heat energy. It is this latter process which has received great attention in recent years due to the shortage and high cost of liquid petroleum fuels.
A typical fluidized bed reactor comprises a quantity of solid granulated material, or bed, disposed above a perforated plate through which the fluidizing gas is forced. The plate functions to both support the bed of solid material and to distribute the fluidizing gas evenly throughout the reactor. By controlling the velocity of the gas passing through the perforated plate, the bed of granulated material is maintained in a "fluidized" state and constitutes the reaction zone. The range of acceptable velocities for fluidization is dependent upon the size of the individual granules of solid material and the fluid transport properties of the fluidizing gas.
As with any continuing reaction, fresh solid reactant must be continuously supplied and the products of reaction continuously removed. Fresh solid reactant is prepared and introduced into the fluidized bed reaction zone by a variety of methods. Typically, the solid reactant is received in a form which is not suitable for immediate introduction into a fluidized bed reactor. In the case of a fluidized bed coal combustion system, coal and limestone are received as mined with little or no advance preparation. The solid materials are introduced into a mechanical crusher or other means for preparing the as-received materials for introduction into the reactor. In practice this preparation results in a stream of solid reactant granules with varying sizes.
When introduced into the fluidized bed reaction zone, that portion of the solid feed material below a certain granular size quickly becomes entrained in the fluidizing gas and is carried out of the fluidized bed reactor. The short period of time in which these elutriated reactants are present within the zone reduces the probability that they will be reacted, thus resulting in a loss of unreacted solid material from the fluidized bed reactor. This unreacted material loss may represent a significant efficiency penalty in the operation of the fluidized bed reactor system.
Current methods of reducing this loss include providing a separate reactor system for reacting the removed particles or reintroducing the removed particles into the main fluidized bed reaction zone. This latter method has not been successful in that the reintroduced material, having previously been elutriated due to its small particulate size, is subject to the same environment upon reintroduction and is likely to be quickly elutriated from the reaction zone again.
A variety of solid reactant feed systems are present in the art which attempt to evenly distribute the solid reactant throughout the fluidized bed reactor and to increase the residence time of the elutriation-prone granules within the fluidized bed. Typical feed systems presented in the art include: underbed feed systems, wherein the feed stream of solid granules is divided into a plurality of feed pipes arranged beneath the perforated plate of the fluidized bed system and rising therethrough discharging the solid feed material directly into the fluidized bed reactor; overbed feed, in which the solid granules are allowed to drop into the fluidized bed under the influence of gravity through a conduit; and stoker feed systems, in which an apparatus for distributing the feed is arranged above the fluidized bed reactor whereby the solid granules are permitted to drop into the fluidized bed reactor under the influence of gravity.
It can be recognized that the use of overbed or stoker feed systems may result in a portion of the solid feed becoming immediately entrained in the fluidizing gas and never reaching the fluidized bed reactor. The problem of undesirable loss of unreacted solid material has also been recognized in underbed solid feed systems as witnessed by attempts in the art to increase the residence time of granulated within the fluidized bed, for example Zielinski, U.S. Pat. No. 4,309,948.
An additional problem with the use of fluidized bed reactors for the purposes of useful heat generation arises from the limited temperature controllability of the fluidized bed reactor. Temperature in a fluidized bed combustion reactor must be maintained within a limited temperature range. Too low an operating temperature results in incomplete absorption of the sulfur evolved during the combustion process, to high an operating temperature results in fusing and agglomeration of inert ash compounds typically present in the solid fuel.
A common temperature for a fluidized bed coal combustion system using limestone as the sulfur absorbing compound and air as the oxygen bearing gas is about 1500.degree. F. This limitation on the range of operating temperatures of the fluidized bed results in a limitation on the controllability of the heat absorption in the heat absorbing surfaces located in the exit gas stream. Methods currently present in the art for controlling this heat absorption include the use of an attemperating sprays in the heat absorbing medium and/or removing a portion of the fluidized bed reactor from service during periods of reduced load operation.
In summary, fluidized bed reaction systems provide an effective and attractive means for reacting gas solid mixtures, but are presently limited in efficiency and applicability due to poor controllability of the heat absorption rates and the short residence time of very small particles within the reaction zone, resulting in the loss of unreacted solid material.